FBI agents from Little Rock, Ark., where the foundation was started, have taken the lead in the investigation and have interviewed at least one witness in the last month, and law enforcement officials said additional activities are expected in the coming weeks.

The officials, who spoke only on condition of anonymity, said the probe is examining whether the Clintons promised or performed any policy favors in return for largesse to their charitable efforts or whether donors made commitments of donations in hopes of securing government outcomes.

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The probe may also examine whether any tax-exempt assets were converted for personal or political use and whether the foundation complied with applicable tax laws, the officials said.

One witness recently interviewed by the FBI described the session to The Hill as “extremely professional and unquestionably thorough” and focused on questions about whether donors to Clinton charitable efforts received any favorable treatment from the Obama administration on a policy decision previously highlighted in media reports.

The witness discussed his interview solely on the grounds of anonymity. He said the agents were from Little Rock and their questions focused on government decisions and discussions of donations to Clinton entities during the time Hillary Clinton led President Obama's State Department.

The FBI office in Little Rock referred a reporter Thursday to Washington headquarters, where officials declined any official comment.

Foundation spokesman Craig Minassian took a more muted response, saying the new probe wouldn't distract the charity from its daily work.

“Time after time, the Clinton Foundation has been subjected to politically motivated allegations, and time after time these allegations have been proven false. None of this has made us waver in our mission to help people," Minassian said. "The Clinton Foundation has demonstrably improved the lives of millions of people across America and around the world while earning top ratings from charity watchdog groups in the process."

The Wall Street Journal reported late last year that several FBI field offices, including the one in Little Rock, had been collecting information on the Clinton Foundation for more than a year. The report also said there had been pushback to the FBI from the Justice Department.

A renewed law enforcement focus follows a promise to Congress late last year from top Trump Justice Department officials that law enforcement would revisit some of the investigations and legal issues closed during the Obama years that conservatives felt were given short shrift. It also follows months of relentless criticism on Twitter from President Trump, who has repeatedly questioned why no criminal charges were ever filed against the “crooked” Clintons and their fundraising machine.

For years, news media from The New York Times to The Daily Caller have reported countless stories on donations to the Clinton Foundation or speech fees that closely fell around the time of favorable decisions by Clinton's State Department. Conservative author Peter Schweizer chronicled the most famous of episodes in his book "Clinton Cash" that gave ammunition to conservatives, including Trump, to beat the drum for a renewed investigation.

Several GOP members of Congress have recently urged Attorney General Jeff Sessions to appoint a special counsel to look at the myriad issues surrounding the Clintons. Justice officials sent a letter to Congress in November suggesting some of those issues were being re-examined, but Sessions later testified the appointment of a special prosecutor required a high legal bar that had not yet been met.

His decision was roundly criticized by Republicans, and recent revelations that his statement was watered down by edits and that he made the decision before all witness interviews were finished have led to renewed criticism.

A senior law enforcement official said the Justice Department was exploring whether any issues from that probe should be re-opened but cautioned the effort was not at the stage of a full investigation.

One challenge for any Clinton-era investigation is that the statute of limitations on most federal felonies is five years, and Clinton left office in early 2013.